This invention generally relates to methods for registering computer-generated images. In particular, the invention relates to the registration of images in an ultrasound imaging system.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line""s resolution is a result of the directivity of the associated transmit and receive beam pair.
A B-mode ultrasound image is composed of multiple image scan lines. The brightness of a pixel is based on the intensity of the echo return from the biological tissue being scanned. The outputs of the receive beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed and scan-converted to form an image frame of pixel data which can be displayed on a monitor.
Multiple scans are performed in succession and multiple image frames are displayed at the acoustic frame rate on the display monitor. In the case where the sonographer is manipulating the transducer probe by hand, any change in the angle (i.e., wobble) of the probe during scanning will cause corresponding changes in the angle at which the transmit beam impinges on the biological tissue being imaged. The result is that some tissue will appear bright during one scan at one angle and dark during another scan at another angle. To compensate for this angle-dependent fluctuation in the intensity of the echo signals reflected by the tissue, it is well known to combine the bright regions from successive image frames to form a compound image. This process is known as spatial compounding. The key to successful spatial compounding of the ultrasound images from different angles is to accurately register the ultrasound image frames. Errors in image registration during compounding can cause blurring and possibly loss of the details in the original images.
Several methods have been used successfully in ultrasound image registration, including the minimum sum of absolute differences (MSAD) technique, the correlation technique and the landmark matching technique. These techniques are based on spatial domain analysis of image features and may be sensitive to noise. The MSAD technique, which might be the best among these spatial domain techniques, has the advantages of being simple and fast, but it is not accurate in rotational registration, especially for large rotation angles. The correlation technique is computationally very expensive. Also the correlation technique is prone to error for large rotation angles. The landmark matching technique has limited application in ultrasound image registration because it is difficult to do automatic registration using this technique. Furthermore, it is difficult to deal with scaling (dilation) using any spatial domain technique.
Thus there is a need for an ultrasound image registration technique which is not afflicted with the disadvantages of the spatial domain techniques.
The present invention is a method and an apparatus for registration of frames of computer-generated images (hereinafter xe2x80x9cimage registrationxe2x80x9d). Although the preferred embodiment is an ultrasound imaging system in which image registration is used to prepare multiple images for spatial compounding or for three-dimensional reconstruction, the invention also has application in other imaging modalities, such as computerized tomography and magnetic resonance imaging.
The preferred embodiment of the invention using a technique which will be referred to herein as xe2x80x9cFourier phase matchingxe2x80x9d. This Fourier phase matching technique is performed by a computer after a multiplicity of image frames of pixel intensity data have been acquired and stored, e.g., in cine memory. The computer is programmed to perform an algorithm on the data of these image frames to achieve image registration prior to spatial compounding or three-dimensional reconstruction, either of which may also be performed by the same computer. The computer may be the system host computer or a dedicated processor. In accordance with the preferred embodiments of the invention, the image registration algorithm comprises a Fourier phase matching technique.
The image registration technique in accordance with the preferred embodiments translates, rotates and scales each image frame of a succession of frames to achieve registration with the previous image frame. This Fourier phase matching technique is based on the frequency domain matched filter (for phase) concept, so it is simple to implement, accurate and less affected by image noise. This method has been used for optical image registration in the prior art.
In accordance with one preferred embodiment, a series of consecutive image frames are retrieved from the memory of an ultrasound imaging system. The image frames are selected by the system operator using an operator interface, e.g., a trackball control. Each selected image frame is registered with the previous image frame using the image registration algorithm. After image registration, the registered frames of pixel intensity data are compounded, e.g., by selecting the maximum (i.e., peak) value of the pixel values corresponding to a particular pixel position in the respective registered image frames or by calculating the mean (i.e., average) of the pixel values corresponding to a particular pixel position in the respective registered image frames. If the maximum pixel value is used, an appropriate normalization may be performed to ensure that the compounded image has a desired overall intensity level when displayed.